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Professional Paper 1715

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Petrology of the Crazy Mountains Dike Swarm and Geochronology of Associated Sills, South-Central Montana

By Edward A. du Bray, Stephen S. Harlan, and Anna B. Wilson

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The Crazy Mountains dike swarm is a radial array associated with the Eocene (approximately 49 Ma) Big Timber stock in the Crazy Mountains of south-central Montana. Dikes are hosted by the stock and by Paleocene Fort Union Formation rocks that host the stock. The spatial coincidence, field relations, and similarity of petrographic and geochemical features suggest that the dikes and the stock are cogenetic and coeval. The dikes depict a broad range of compositions, from rhyolite to basalt; dike composition is only weakly related to position relative to the Big Timber stock. The dikes range from 51 to 71 weight percent SiO2, a slightly more restricted range than that exhibited by the stock. Compositional variation among the dikes is less systematic than is characteristic of the stock; dike compositions form more broadly diffuse arrays and tend to be slightly more alkaline. Both of these factors could result from late-stage magmatic-hydrothermal processes that seem to have weakly altered many of the dikes. Major-oxide and trace-element characteristics of the dikes and the associated stock are consistent with a mantle-dominated genesis in a subduction-related volcanic arc setting. In addition to petrologic and geochemical data, new 40Ar/39Ar geochronologic data for strongly alkaline sills in the northern Crazy Mountains are also presented. Dates for the sills, which are peripheral to the Crazy Mountains dike swarm, indicate that alkaline magmatism was temporally associated with subalkaline magmatism represented by the Big Timber stock and related dikes. Characteristics of the subalkaline magmatism are consistent with renewed volcanic arc magmatism during the Eocene westward hingeline retreat and subsequent southwestward migration of the magmatic front in response to steepening subduction along the western edge of North America.

Version 1.0

Posted March 2006

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